Apparatus for electrochemical processing

ABSTRACT

An apparatus for electrochemical processing characterized by extended surface electrodes maintained at a substantially uniform electrical potential while presenting a relatively low pressure drop to liquids in treatment passed through the apparatus.

United States Patent [191 Williams 1 Jan.7, 1975 APPARATUS FORELECTROCHEMICAL PROCESSING [75] Inventor: John M. Williams, Newark, Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

22 Filed: on. 17, 1973 21 Appl. No.: 407,248

Related U.S. Application Data [63] Continuation-impart of Ser. No.290,642, Sept. 20,

1972, abandoned.

2,588,450 3/1952 Zadra 204/109 2,816,067 12/1957 Keidel 204/1303,061,537 10/1962 Yagashita.... 204/130 3,244,605 4/1966 Hotchkiss204/l53 3,457,152 7/1969 Maloney, Jr. et al.... 204/130 3,459,646 8/1969Carlson..... 204/153 3,650,925 3/1972 Carlson 204/284 OTHER PUBLICATIONSJournal of Applied Electrochemistry 2, (1972), pgs 113-l22.

Porous Cathode Cell for Metals Removal from Aqueous Solutions, by G. A.Carlson and E. E. Estep, Advance Copy of paper of ElectrochemicalSociety Meeting, Houston, Texas May 10, 1972.

Primary Examiner-T. M. Tufariello [57] ABSTRACT An apparatus forelectrochemical processing characterized by extended surface electrodesmaintained at a substantially uniform electrical potential whilepresenting a relatively low pressure drop to liquids in treatment passedthrough the apparatus.

9 Claims, 16 Drawing Figures SHEET 30F 6 PATENTED JAN [7 I975 PATENTEUJAN 71975 SHEET 8 BF 6 l'lllll'l APPARATUS FOR ELECTROCHEMICALPROCESSING CROSS REFERENCE TO RELATED APPLICATIONS This Application is acontinuation-in-part of US. application Ser. No. 290,642 filed on Sept.20, 1972 now abandoned.

BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises anapparatus for the electrochemical treatment of an electricallyconductive solution by effecting separation, reaction or otherprocessing of a given ingredient of the solution. The apparatus is anelectrolytic cell comprising at least two inter-functioning electrodesdisposed within a vertically oriented leak-tight housing, at least oneof which electrodes has an extended surface area over which asubstantially uniform reaction-producing electrical potential ismaintained throughout that portion of the area of the electrode inconfrontation with the electrolyzing area of the remaining electrode,the separatory (or reactive) one of the electrode pair being relativelyopenn to access of electrically conductive solutions passed through thecell, whereas the remaining electrode is isolated frorm the solution byenclosure within an electrically insulative envelope permitting ionicconduction while barring ready passage of the soluution therethrough.

DRAWINGS The invention is described with reference to the followingdrawings, in which:

FIG. 1 is a partially schematic side elevation sectional view through afirst (cylindrical) embodiment of cell constructed according to thisinvention,

FIG. 2 is a block diagram of an arrangement of apparatus effectingseparation of heavy metal from a dilute electrically conductive solutionas process liquid,

FIG. 3 is a somewhat schematic partially opened view of a preferredembodiment of spiral wrap electrode structure for use with thisinvention,

FIGS. 4A-4C are partially schematic representations of two differentknitted and woven electrode element designs of uniformly reticulatedopen construction which can be utilized with this invention,

FIG. 5A is a partially schematic side elevation view of a thirdelectrode structure of cyclone fence" design,

FIG. 5B is an end view of the structure of FIG. 5A taken on lines 5B-5Bthereof, 7

FIG. 5C is a partially schematic fragmentary perspective view of afourth electrode structure fabricated from expanded metal,

FIGS. 6A and 6B are, respectively, diagrammatic flow sheets ofcontinuous mode operation and of batch mode operation with solutionrecycle according to this invention,

FIG. 7 is a plot of copper removal according to this invention whereinCu content in ppm is plotted as ordinate versus pass number through theapparatus as abscissa,

FIG. 8 is a plot of copper removal according to this invention from atypical industrial effluent containing Cu as adulterant wherein Cuconcentration is plotted in ppm as ordinate versus time in minutes asabscissa,

FIG. 9 is a partially schematic perspective view of a rectangularelectrode embodiment of this invention.

DETAILED DESCRIPTION This invention will be described primarily asapplied to pollution abatement in the cathodic removal of heavy metalsat low concentrations from dilute electrically conductive solutions, orwaste effluents; however the invention is equally applicable to highconcentration uses, anodic processing and also the effectuation ofelectrochemical reactions, such as oxidation and the like, all ashereinafter detailed.

In ordinary electrochemical processing of aqueous solutions, e.g., inelectroplating, the ionic components are present in relatively highconcentrations and thus migrate to the electrodes at rates consistentwith practical current densities and at relatively high coulombicefficiencies for the desired reactions. However, in many situations,such as most aqueous wastes from industrial plant operations, the ioniccomponents are present at levels two or three orders of magnitude lowerthan those encountered in plating practice. If electrochemicaltreatments are to be applied to the low concentrations of contaminantsin such aqueous wastes, it is found that conventional apparatus known tothe art possesses very low electrical efficiency, coupled with excessivecell residence time, which renders it impractical to reduce theconcentration of the contaminants to the safe levels prescribed by law,e.g., one part per million.

Research on conventional electrochemical cell operation reveals that,when the concentrations of species involved in electrochemical reactionsare very low, other competing electrochemical reactions canconcomitantly ensue, proportionately reducing the efficiency of thedesired reactions. Thus, in aqueous metal solutions, incidentalproduction of hydrogen at the cathode instead of plateout of the desiredmetal component, such as copper, constitutes electrical current waste.In some instances objectionable mixed metal deposits are also produced,instead of the desired pure metal, whereas, in other instances, thedeposits are of inferior physical character, such as loose powdersrather than adherent films.

Attempts have been made to solve the problems described by utilizingporous carbon electrode equivalents, including fixed and fluidized bedsof carbon particles surrounding the metallic bus supplying current,thereby relying only on point-to-point contact of the carbon particlesto maintain conduction. Electrodes fabricated from porous carbon havealso been used, through the pores of which the liquid electrolyte isforced to flow; however, these suffer from high pressure drop as well asrapid blinding resulting from accumulation of the plated-out componentwithin the fine pores.

Blocks of carbon can be arranged to provide a high area electrodesurface per unit volume of the bed; however, there are disadvantages,such as: (1) carbon has a higher specific resistivity than most metals,so that an electron current flowing within a solid carbon matrixdisplays a substantial voltage drop between the supply bus and theremote regions of electrode surface. Nonuniformity of electrode surfacepotential permits concomitant multiple electrochemical processes atdifferent electrode regions, instead of restricting operation to adesired single process. (2) Conventional porous carbon has a very finepore size of, typically, 0.001 inch-0.006 inch (25 to 150 u) and is,therefore, quickly clogged with an electrochemical deposit. (3) Fluidflow resistance is very high as a result of both fine pore size and lowporosity (e.g., typically, the void fraction is 0.5 or less), requiringeither high pressure operation or a very low flow rate.

Packed beds of carbon particles can, by careful selection of particlesize and shape, afford a more open deposit area for the deposition ofelectrochemical product, which has a longer servce life, beforeblinding, together with a lower pressure drop. However, a new difficultyis encountered, namely, reliance on point-topoint' particles contact forthe electrode circuit path, which evinces a higher electrical resistancebetween the supply bus and the bed extremities, so that currentefficiency is reduced even below that of a block carbon electrodestructure. Moreover, the void fraction of packed beds is usually onlyabout 0.50, making only half of the bed section available for passage ofthe solution in'treatment.

Fluidizedbeds of carbon particles display even more severedisadvantages, in that: (1) there is a substantially higherparticle-to-particle electrical resistance, because the area of particlecontact are smaller and there exist only low interparticle pressure,since the bed is at least partially floated by the process streamthroughput, (2) part of the time some particles are completely out ofcontact with any others, contributing zero electrode activity and, wherecorrosive solutions are being treated, actually reducing overallefficiency by loss of previously deposited material through corrosiveattack, and (3) deposits built up on individual fluidized particleschange the fluidization properties of the particles, making forobjectionable inconstancy and nonuniformity of electrode behavior.

Use of metallic extended surface electrode structures is disclosed inU.S. Pat. No. 2,588,450 in the form of a loose stainless steel wool padas cathode disposed within a cylindrical basket fabricated from aninsulative material which is perforated on the periphery to divert theelectrolyte flow to an anode disposed coaxially with respect to thecathode. The cell is utilized for the electrodeposition recovery of goldfrom caustic solution, and the electrolyte is introduced first into thecenter of the cathode, after which it flows generally laterally throughthe basket side wall and thence to the encircling anode, so that Contactof electrolyte with the electrodes is sequential from cathode to anode.

US. Pat. No. 3,244,604 shows electrolyte flowthrough woven mesh cathodesA to 4 inches thick, used for removing metal ion impurities which arepresent at concentrations below about 500 ppm in aqueous causticsolutions. The cathodes are pads of woven nickel wire stiffened by anickel screen to which electrical connection is made. Anodes andcathodes alternate in vertical array in the direction .of electrolyteflow and there is, of course, no separator inhibiting electrolyteflow-through with respect to the anodes, since the electrolyte has toflow with equal facility through all of the electrodes in the verticallydisposed stack.

Neither of the disclosed constructions employ simultaneous subjection ofelectrolyte to both anode and cathode operation at common transverselevels, and this is an important feature of this invention.

THIS INVENTION For the conduct of electrochemical reactions, theessential electrical potential of a point in an electrolyte, hereinafterreferred to as the reactionproducing potential," is equal to the sum ofthe following:

1. The equilibrium reversible half-cell potential at the conditions ofthe electrolysis (i.e., temperature, pressure, concentrations of speciesat the electrodeelectrolyte interface, etc.),

2. The activation overpotential, this being the extra electricalpotential over and above the equilibrium reversible half-cell potentialrequired to drive the desired reaction at a given rate. Thisoverpotential is a function of the real current density at theelectrode-electrolyte interface, and

3. Ohmic voltage drop in the electrode material. This voltage drop isthe integral sum of the ohmic voltage drops along each of the paths ofcurrent flow leading from the electrode-electrolyte interfaces to thesaid point in the electrode.

This invention advantageously influences all three of the factorsdeterminative of electrode point potential in the following respects:

a. The electrodes, constructed of extremely fine filaments which providelarge surface area and enhance the mass transfer coefficient, minimizethe concentration difference between the bulk of the electrolyte and theelectrode-electrolyte interface at any given level of operation.correspondingly, the departure from the equilibrium reversible half-cellpotential predicted from the bulk concentrations is minimized.

Accordingly, in a situation where competing reactions can occur, thepotential region corresponding to this undesirable condition is lessclosely approached, so that the specificity, or coulombic efficiency,for the desired reaction is enhanced.

b. The activation overpotential, which is a function of the currentdensity, is minimized at any given level of operation by the high realsurface area of the extended surface electrode. This contributes evenmorre importantly than (a) supra to selectivity and enhanced coulombicefficiency for the desired reaction.

c. The high conductivity of the preferred electrode materials, togetherwith the use of welded or other low resistance connections to buses,minimizes ohmic voltage drop for any given level of operation.

By reduction of each of the enumerated voltage contributions, the powerrequired for a given level of cell operation is substantially reduced. Afurther contribution to higher power efficiency is the closejuxtaposition of anode to cathode in cells of this invention. This isachieved by enclosing one of the interfunctioning electrodes of the pairwithin a separator presenting both sides of this'isolated electrode inelectrolyzing disposition with respect to a co-functioning electrode.

Referring to FIG. 1, a cylindrical embodiment of electrochemical cellaccording to my invention comprises the high surface cathodic designshown, which is here utilized to remove heavy metals (e.g., Cr, Mn, Fe,Ni, Cu, Zn, Mo, Ag, Cd, Co, Hg and Pb) from dilute aqueous solution. Inthis service, only a relatively small anode is required and this can,accordingly, constitute a compact rolled-up metal screen 10 which iscentrally located along the longitudinal axis of the cell enclosurewhich, in this instance, can typically be a tube 11 fabricated fromglass, polymeric resin or the like. If desired,

a metallic enclosure 11 can be employed, provided that the anode supplybus 12 is electrically insulated therefrom, in which case the insidewall surface of the enclosure constitutes additional cathodic area.

The anode supply bus 12 of FIG. 1 is introduced in fluid-tightrelationship through a passage drilled radially in the wall of enclosure11 and is bent downwardly into firm contact with adjacent plies of thescreen over the full length thereof, preferably being weld-attachedthereto.

The cell of FIG. 1 is provided with two concentric cathode elementsleeves l4 and 15 coaxially disposed with respect to the common axis ofenclosure 11 and anode 10, although a greater or lesser number can beutilized if desired. An essential feature of this invention is theutilization of cathode elements which have a very large extendedsurface, while at the same time insuring substantially equipotentialmaintenance over the electrochemically effective surface as well aspresenting an open structure permitting low pressure drop passage ofelectrolyte therethrough. Bulky open mesh designs of the structuresshown in FIGS. 4A and 4B have proved highly effective because of theiruniformity in construction and because of their physical flexibility inassembling apparatus such as those shown in FIGS. l and 3. However,uniformly reticulated structures generally, such as the species shown inFIGS. 4C and SA-SC, can also be used.

Extended surface electrodes employed in my cells can be made from evenlydistributed highly conductive continuous run corrosion-resistant metalfilaments 2 to 4.5 mil dia., fabricated into knitted form having a voidvolume in excess of about 85 percent and a surface area in excess ofabout 5 cm /cm of electrode volume. While FIG. 1 details extendedsurface cathodes, it will be understood that, where anodic electrodesare to be utilized in electrochemical processing, the same electrodestructures are ideal for anodic service and, in fact, the electricalsupply leads can be simply reversed in polarity to convert cathodicoperation into anodic operation. Cathode bus connections are effectedthrough branched upright bus connectors 17a, weld attached to radiallyopposed pairs of cathodic elements l4, l5 and attached at the bottomends to a cathodic bus 17 extending radically out through the wall ofenclosure ll to a suitable conventional d-c source, not detailed.

As shown in FIG. 1, anode Ill is mounted within a spacer cage 20, whichcan be of generally open cylindrical form constituting circular strips20a joined on the inside surfaces to upright strips 20b, pairs of whichlatter elements define vertical passages between them for escape of gasreleased at anode 10. A porous construetion is preferred for spacer cage20, a preferred material of construction for an electrically insulatingcage being a unitary bulky polymeric product (e.g., polyethylene)produced by extrusion through a rotating grooved annular extrusion die.The producct is an open net-like structure made up of coarse filamentslaid over one another at right angles and bonded at crossing points.

Optionally, an electrically conductive (metal) spacer cage of generallysimilar construction as that hereinabove described can be employed, inwhich case cage 20 constitutes a radial extension of anode it) since itis in conductive contact therewith.

Enclosing the anode-spacer cage subassembly is a porous cup-likenonconducting electrode separator 23 having its base end 23a disposed inthe direction of process liquid input which, in FIG. 1, is shown as fromthe bottom as denoted by the directional arrows. Separator 23 can betypically fabricated from spunbonded polyethylene.

The purpose of separator 23 is to forestall electronic conductionbetween anode l0 and cathode 14, 15 by isolating against metal-to-metalelectrode contact, while still furnishing an ionic passage through thesolution spanning the electrodes. The permeability of separator 23should be moderately high for good ionic conductivity while still beinglow enough to prevent gross mixing between the anolyte and catholytesolutions.

It is also permissible to fabricate separator 23 from electricallyinsulative ion-exchange membrane materials, such that ions of at leastone component in the solution can pass through the separator, therebycompleting the ionic current path, while forestalling directmetal-to-metal electronic conduction between the electrodes within thecell as well as direct solution flow between electrolyte chambers.

In summary, the cell structure described supra utilizes very largeextended surface electrodes arranged so as to (l) accommodate liquidelectrolyte flow therepast with only moderate pressure drop, (2) provideshort path distance from any point in the electrolyte in process to theeffective extended surface electrode in order to facilitate rapid ionicdischarge and (3) insure exceedingly uniform potential maintenancebetween bus connectors 17a and cathodes 14, 15 by keeping the electricalresistivity to a minimum not only within the electrode structure per sebut also from point-to-point throughout the electrode structure en routeto buses 17.

The absolute magnitudes and relative quantitative importance of thephysical parameters of the extended surface electrodes described depend,importantly, upon the particular electrochemial process to be effected.For example:

1. In the cathodic deposition of heavy metals such as Cu existing in lowconcentrations in dilute H solutions, the current densities required forstoichiometric equivalence to the metal content removed, withconventional liquid flow velocities through the cathode, will berelatively low. The voltage drop across the conductive path betweensupply bus 17, bus connectors 17a and the extremity of any filamentarypath in the cathode mesh is the product of current (I) and pathresistance (R). If fine filaments are used in the cathode structure inorder to secure large surface per unit volume of the mesh, the increasein R resulting from the small diameter filaments is largely offset bythe high conductivity. Thus, the existing IR drop is relatively minimal,so that the voltages at all points on the cathode surface areapproximately the same as the supply voltage of bus 17 and busconnectors 17a.

2. In the cathode removal of one metal while preserving an accompanyingmetal of nearly equal deposition voltage in solution, it becomes farmore important that in (1) supra to assure a close approach toequipotential cathode surfaces, and filaments of heavier gage arepreferred in this regard.

3. When the concentrations of materials being treated are higher than inthe usual wastes, e.g., in electrowinning, the operation is lessaffected by rate of mass transfer of ions to an electrode because moreions are available and the current densities are higher in meeting thestoichiometric requirements, so that it becomes even more important thatin (1) and (2) supra to assure equipotential electrode surfaces. Here itis often feasible to sacrifice some electrode open fluid space,concomitantly increasing the conductive filament cross-sections.

4. As an extension of case (3) supra, in situations where no depositforms on the electrodes, e.g., anodic oxidation or cathodic reductionforming soluble products, likelihood of plugging porous electrodes isgreatly reduced, therefore it is practicable to substitute lowerporosities in favor of heavier electrode cross-sections.

Referring now to FIG. 3, there is shown a preferred design of cellaccording to this invention employing one-piece spirally wrappedcathode, anode electrode structures which are particularly advantageousin providing a large electrode surface in form adapted to insertion intoa tubular, leak-tight housing, such as a conduit. Elements in FIG. 3corresponding to the same elements in FIG. 1 are denoted by the samereference numerals, except that these are primed. The external housing11 is omitted in the showing of FIG. 3 and, to more clearly expose theanode-spacer structure, the outer terminal portions of the interwrappedelements are pulled apart radially to bring sizable surface portion intobetter view.

The cathode, or outer member shown in FIG. 3 is portrayed as havingthree schematically represented cathode elements, i.e., 14,15, and 25,each consisting of a flattened stainless steel wire tubular knitstructure pressed into tight electrically conductive contact with itsneighbor. Also, since anode which can be a screen or foil ofelectrochemically inert material (e.g., platinum), is effective on bothof its side surfaces, individual spacers and 20" are employed onopposite sides of the anode, the pair constituting the equivalent ofspacer cage 20, FIG. 1. The equivalent of cup-like separator 23 is thesharply folded double ply element 23bottom, which encases thesubassembly 20, 10, 20 on the bottom edge and both sides, it beingunderstood that the process liquid again is supplied from the botton, asdenoted by the directional arrow, whereas exit of treated liquid and anygaseous products is via the top.

Positive supply bus branches l2, 12 are advantageously spaced along thelength of anode 10, to better distribute the supply of electrolyzingcurrent and voltage in accordance with the demand imposed by equalincremental lengths of anode surfaces. The same principle is appliedwith respect to negative supply bus connectors 17a, 17a. The structureof FIG. 3 is shown in somewhat loosely rolled condition, so that thereexists a longitudinal passage 26 at the inner extremity of the spiral.With tighter roll wrapping, this bore virtually disappears, or can beplugged by a non-conducting rod, so that no bypass through the cell ispresented to process solution flowing axially thereof.

The structure of FIG. 3 can be neatly fitted into a circular tubeenclosure and, even though it is tightly wrapped into very compactspace, only moderate resistance is interposed to solution flow.

Referring now to FIGS. 4 and 5, there are shown details of extendedsurface electrode formation. Thus, FIG. 4A (and end view 48) show aknitted bulky wire structure, such as used in the apparatus of FIG. 1,made by interlooping adjacent courses of long length wire denoted 27,which are weld-joined to bus connectors 17a.

Very satisfactory operation has been obtained using cathode meshstructures of the configuration shown in FIGS. 4A and 4B employing fromabout twenty to about one hundred and twenty individual wire layers. Itis a convenience to utilize flattened tubular knit sleeves fabricatedfrom Type 316 stainless steel wire having two filaments in each loop,the filament diameters being any of2 mil, 3 mil or 4.5 mil(corresponding, respectively, to 0.051 mm, 0.076 mm and 0.115 mmdiameters), as marketed commercially by Mctcx Cor poration, Edison, NJ.Referring to FIG. 3, and utilizing a single spacer made up of Du PontVexar polymeric netting measuring about 15 mils (0.38 mm) thick and aseparator 23 measuring about 6 mils (0.152 mm) thick, it was practicableto obtain a four-wrap cathode structure, each wrap of which constitutedtwenty-five overlaid double layer flattened knit sleeves, or a total oftwo hundred radially disposed individual mesh layers, which assembly fitsnugly into a Pyrex pipe enclosure having an inside diameter of 51 mm.

A great variety of materials having a wide range of dimensions can beutilized in extended surface electrodes as taught in this invention,however, the following general combination has given exceptionally goodresults: (I) spiral arrangement of electrodes as shown in FIG. 3, (2)use of a pair of spacers 20' (one on each side of the anode) insuringunhindered egress of any gases generated, especially at the anode l0,(3) a separator constituting an envelope open only along the cellelectrolyte outlet side made up ofa strong web material such as ionexchange membranes, textile fabrics, paper and spunbonded productshaving a permeability to ionic passage equivalent to a pore size ofabout 0.2 mil, (4) knitted mesh of continuous metallic filaments asbasis for the extended surface electrode such as shown in FIGS. 4A and4B, (5) electrode mesh size 6-20 courses/inch (2.4-8 courses/cm), (6)extended surface electrode area 30-50 cm lcm (7) ratio of electrode areacathode/anode 5-5O in service of removing trace heavy metals from wastesolutions, (8) void volume in extended surface electrode, 90-95 percent,(9) extended surface electrode filament size, 2-5 mils (50 1.1.), (10)treated solution flow velocity, l-l0 cm/sec, and (11) anode currentdensity, 5-200 ma/cm based on the anode s simple projected area (i.e.,the product of width x height).

Cells constructed and operated as described supra have the followingcharacteristics: (1 average distance from any point in liquid in processflowing through extended surface electrode to a filament surface is 0.7mm., (2) when operated at a high anode current level (e.g., 60 malcm themaximum voltage drop from the bus bar to the most remote location in theextended surface electrode structure is less than about 0.160 volt and(3) the pressure drop through a typical 51 mm. dia. jellyroll spiralelectrode assembly for a mm. active length of the roll of extendedsurface electrode (cathode) is as follows for two sizes of filament inknitted mesh (FIG. 4A design):

TABLEI PRESSURE DROP VS. FLOW RATE In FIG. 4C there is shown a Turkishtowel-like wire cloth mesh structure having continuous conductivefilaments 30 and 31 forming a looped pile extending outwardly from bothsides of the woven structure. The ends of filaments 30 and 31 aresecurely bonded to the bus connector 17a by welding, brazing or the likefor low electrical resistance.

In this structure the loops are woven in as part of the warp andconstitute the wire structure of the electrode. Filler fibers 32 run atright angles to the loops and can, or need not, be electricallyconductive. The purpose of the filler fibers is primarily to completethe weave and hold the entire structure together for mechanicalintegrity. If filler fibers 32 are conductive, it is preferred thattheir ends be joined to an additional bus structure in a manner ashereinbefore described for filaments 30 and 311 in order to insurelowest possible electrical resistance within the cathode structurematerials. Warp filament 33, and others not shown but reversed inorientation with respect to opposite individual loops, are companionatethreads binding the filler fibers 32 in place. If the warp filament 33is conductive, it is preferably welded at the end to bus connector 17a.However, it need not be conductive and, in this case, circuit contactwith the bus is dispensed with.

If a cut pile is employed instead of a loop pile, the entire structureis preferably electrically bonded together by a dip soldering orbrazing.

FIG. A, a plan fragmentary view, and FIG. 58, an end view, together showa cyclone-fence type electrode structure made up of helically interwoundadjacent metal filaments, such as 39a, 39b, weld-joined at their ends tocathode bus connector 17a. The relatively large diameters of the helices(refer FIG. SB) constitute, in multi-layer assemblages, an exceptionallyopen electrode structure not readily plugged by suspended solids in theelectrolyte in process.

Finally, FIG. 5C shows, in perspective, a conventional expanded metalelectrode structure in which the sheet 40 is first provided with a lineof slits and thereafter pulled longitudinally to open the slits intodiamond apertures 41. The outwardly projecting ends are weldjoined tocathode 17a and successive sheets 40 are creased or corrugatedtransversely along lines 42 to furnish the desired separatory offsetbetween adjacent sheet members.

The foregoing description is particularly directed to the preferredembodiment jelly-roll, or spiral assembly, cell construction; however,extended surface flow through electrodes having high conductivity andporosity can equally well be assembled in flat stack form, resembling aclub sandwich arrangement. Thus, this invention is readily applicable tocommercial plateand-frame electrolysis cell designs and the like.

Referring to FIG. 9, there is shown a cell embodiment according to thisinvention utilizing planar electrodes which, in this instance, aredisposed in vertical coparallel rectangular array with the solution tobe sub- 5 jected to electrochemical processing introduced from thebottom, as indicated by the flow directional arrow. The containmenthousing is omitted in this FIG. for simplicity in the showing as is alsothe right-hand cathode sub-assembly, it being preferred to associateeach anode with a pair of cathode assemblies, one on each side.

The anode is, in this instance, a solid metal plate, typicallyfabricated from platinum about 25 mils thick. Anode 70 is disposedwithin a separator envelope 71 of good ionic permeability, ashereinbefore described, which nevertheless effectively isolates theanode from ready circulation of electrolyte liquid through the anoderegion. Spacers 72, also constructed as hereinbefore described, areinterposed between each face of anode 70 and the inside faces ofseparator envelope 71.

Electrical conductor tabs 70a, welded, brazed or oth erwise secured ingood electrical conductive joinder with anodes 70, afford circuitconnection points with an anode current supply bus, not detailed.

The cathode sub-assemblies denoted generally at 75 are here shown, forsimplicity, as each made up of only four contiguous layers of wire meshconstruction as hereinbefore described (e.g., two superposed pieces ofknitted stainless steel sleeve construction each providing a doublelayer cathode element, electrical conduction being maintained throughoutby tight physical contact between adjacent layers). Stainless steelplates 76, typically, 40-60 mils thick, each having an upstanding tab76a for electrical connection with a negative polarity current supplybus 17, not shown, are provided on the sides of the cathodesub-assemblies remote from the associated anodes, thereby completing'theoverall cell assembly.

Referring to FIG. 2 there is shown a schematic flow diagram fortreatment of a typical industrial waste stream such as, for example, adilute aqueous sulfuric acid solution containing small quantities ofcopper sulfate, which, in this case, is advantageously initiallyaccumulated in hold-up tank 45. A pump 46 propels the waste streamthrough an extended surface electrolysis unit of this invention, denotedgenerally at 47, from whence treated product is exhausted to outfall, orother desired destination, via discharge line 48.

For purposes of simplification of the following de' scription, it isassumed that unit 47 is a single cell. Then, after a given period ofoperation, depending on the size of the cell of unit 47 and the quantityof metal which is removed from the waste solution, the cathode space ofthe cell becomes partially filled with the deposit of the metal whichplates out upon the elements of the cathode structure. This increasesthe pressure differential across the cell to a point where it iseconomical to remove the cell from service briefly for regeneration.This is conveniently effected by the balance of apparatus shown in FIG.2.

Thus, a moderately concentrated acid solution, such as nitric acid, isstored in leachate tank 51 so that, when electrolysis is halted in unit47, the acid is forced, by pump 52, through the electrolytic cell ofunit 47 with return back to tank 51. The leaching solution speedilydissolves plated-out metal from the cathode structure to produce ahighly concentrated acid leachate solution thereof. The leachatesolution can be accumulated in tank 51 until a convenient time arrivesto subject it to electrolytic recovery in a conventional electrolyticrecovery unit, denoted generally at 54, to which the solution issupplied via pump 53. The acid, stripped of its metal content, isrecycled back to tank 51 via line 56 for repeated regeneration service.The conventional direct current power supply, denoted generally at 58,

is shown as furnishing electrolyzing current independently to both units47 and 54.

Others methods exist for removing accumulated metal from the extendedsurface electrode of unit 47. Thus, a plugged cell can be taken out ofservice and a fresh cathode substituted for the filled cathodestructure, after which operation is restored. Then, in a separatesystem, the clogged electrode is made anodic with respect to anotherelectrode immersed in a small volume of concentrated electrolyte, andmetal is anodically dissolved away to produce a highly concentratedmetal solution resembling the leachate solution.

It should be mentioned that the electrolytic cells of this invention arewell-suited to multiple use and, in this connection, can be readilyemployed in either series, parallel or combination series-parallelliquid flow convention as dictated by the circumstances. Thus, in oneinstance where the waste stream involved has a very large volumetricflow rate, a multiplicity of small-sized cells were employed in parallelconnection in preference to designing a single large cell to carry theentire load. similarly, in cases where a considerable quantity of metalis to be removed from a waste solution, it can be convenient to treatthe waste stream by series flow through a succession of extended surfaceelectrode cells, the first of which, in order, remove the largequantities of the metal whereas later ones in the series clean up thetraces.

The series arrangement described has the advantage that the first cellsgradually become loaded with metal, and can then be removed forregeneration, while later cells in the series chain take over theburden. Fresh cells progressively added at the end of the chainconstantly maintain the metal removal capability; however, theparticular point of maximum metal removal shifts progressively along thecell chain during prolonged operation.

The electrical supply for the hereinabove described cell arrangements iscompletely independent of the liquid flow conventions. Thus, if multiplecells are operated in liquid flow parallel, all of the potentialsrequired for equal cell plateout would be the same, so that electricalparallel power supply is advantageous. Conversely, if the cells arearranged in liquid flow series, each separate stage usually has aparticular optimal operating voltage, in which case separate powersupplies of preselected voltage output are usually preferred.

In the following examples, two general modes of operation were utilized:(1) single pass and (2) recirculation.

Referring to FIG. 6A, the single pass mode utilized a solution supplyvessel 60 from which solution to be treated was withdrawn via pump 61and routed to the extended surface (ESE) electrolysis cell denoted gen--erally at 47, with discharge therefrom into a receiving vessel 62 fromwhich solution samples for analysis could be withdrawn at will viastopcock 63.

Referring to FIG. 6B, the recirculation mode utilized a solution supplyvessel 60 from which solution to be treated was withdrawn via pump 61'and routed to the extended surface electrolysis cell denoted generallyat 47", with recirculation therefrom, via line 64, back to supply vessel60. Stopcock 63', connected with vessel 60', permitted drawoff ofsolution samples for analysis as desired.

EXAMPLE I A cell was assembled as detailed in FIG. 3 incorporating anextended surface cathode, a polymeric separator, two polymeric spacersand a screen anode layered together as a sandwich-like stack which wasthen rolled into a spiral for insertion in tubular methyl methacrylatepolymer tube.

For this test the cathode (14',l5' and 25) was knitted sleeve material0.002 inches (50.8 microns), 2- filament, S.A.E. 316 stainless steelmesh having ten courses/in (3.9/cm). The separator 23' was Du Pont Tyvek1058, a spun-bonded high-density polyethylene of 1.6 oz/yd (0.0054 gm/cmabout 6 mils (152 p.) thick, with individual fibers in the range of 0.2mil (5 p.) dia. The spacers 20 and 20" were Du Pont Vexar 30 CD8 89, anextruded netting of high density polyethethylene, having a thickness of30 mils (0.76 mm) with eight strands/in. crossing each other at a angle,giving a diamond pattern. The anode 10' was 80 mesh (31.5 mesh/cm) wovenplatinum screen having individual filaments 0.0042 inch (107 fL) dia.

The cathode was made up of sixty folds of sleeve 2.5 inches (6.35 cm)wide (i.e., four pieces, each folded 15 times constituting 120individual layers), weighing a total of 60 gms.

The anode dimensions were 2.5 (6.35 cm) wide X 6 inches 15.2 cm) long.The two Vexar spacers were the same length as the anode, but 2.75 inches(7.0 cm) wide. A piece of Tyvek 7 (17.8 cm) long X 6.0 inches (15.2 cm)wide constituted the separator. It was folded around the spacers and theanodes and then placed on the layered cathode. The electrodes andinterleaved components were then rolled into a tight spiral and insertedinto the 2.0 inches (5.08 cm) inside diameter methyl methacrylatepolymer tube.

The solution treated was a synthetic aqueous waste" having a startingconcentration of 20 ppm by weight of copper (added as copper sulfate)and sufficient sulfuric acid to give a pH of 1.0. The effluentconcentration of copper was determined by atomic absorption spectroscopyafter each pass, with the copper concentration decreasing as tabulatedinfra.

Operation was conducted pursuant to mode 1 (FIG. 6A) except that, aftereach analysis, the treated solution was returned to vessel 60 foranother pass through the cell 47. This cycle was repeated four times. I

Operating conditions for this Example were as fol lows:

The solution was at room temperature and required no temperatureadjustment as a result of the electrolytic treatment.

The solution velocity (superficial) was cm/min, giving a residence timeof 4.2 secs/pass.

Electrical current was supplied at a rate stoichiometrically sufficienton the first pass to remove all copper from the solution, and thiscurrent was maintained at the same lever (2000 ma) throughout allsubsequent passes.

Table 2 Effluent Concentrations Pass No. Copper Concentration (ppm)EXAMPLE II The same cell and synthetic waste employed in Example I wastreated here with operation in the same mode but at progressively higherflow rates and using proportionately higher electrical currents. ThusCurrent Feed Rate, liters/min.

Velocity,

Run cm/sec.

(Example l) 2,000 6,000

ported.

3O EXAMPLE 111 A cell was constructed as described for Example I, exceptthat the cathode consisted of twenty layers (18.1 gms total) of 316stainless steel knitted mesh made from 0.002 inch dia. filaments withtwo strands/- loop and ten courses/inch.

The separator was Tyvek 1058 and the spacer was Vexar PBS 89, apolypropylene netting mils (380p) thick. Due to space limitations, onlyone layer of Vexar spacer was used with the anode in the Tyvek separatorenvelope. As in EXample l the anode was a 2.5 X 6.0 inch piece of 80mesh woven platinum screen. The components were layered into a sandwich,rolled into a tight spiral and inserted into a glass tube 1.0 inch india.

The solution treated was a synthetic waste composed of copper sulfate,sulfuric acid and water (copper content 103 ppm and pH 1.0). The cellwas operated in mode 1 (FIG. 6A) by passing the fresh coppercontainingsolution in approximately one liter quantities at a flow rate of 500cc/min, corresponding to a superftcial velocity of 1.67 cm/sec, whilemaintaining the current supply at the listed levels with the resultstabulated as follows:

Table 3 Copper Conc'n. (PP

Current a) Voltage (volts) EXAMPLE IV A cell was assembled exactly asdescribed in Example [[1, and operated as in Example 1. Two differentalkaline solutions, each containing approximately 20 ppm of dissolvedcopper, were individually treated. Their compositions were as follows:

A. 17.0 ppm of copper from copper sulfate, 2.75 gms/liter of (NH 80,, pHadjusted to 8.0 with NaOH; and

17.9 ppm of copper from copper sulfate, 3.0 gms/liter of (N11,), S0 and1.0 gm/liter of NaCl, pH adjusted to 8.9 with NaOH.

The solution to be treated was pumped from vessel 60 through cell 47(mode 1) at a flow of 300 cc/min. (1.0 cm/sec superficial velocity) inrepetitive passes and current was supplied at 300 ma. The effluentcopper concentration was determined by atomic absorption spectroscopyafter each pass through the cell and is tabulated as follows for the twosynthetic waste solutions:

Table 4 Effluent Concentrations from Alkaline Electrolysis Pass SolutionA Solution B No. (Cu in ppm) (Cu in ppm) EXAMPLE V A cell wasconstructed as described for Example Ill, except for the following: SAE304 stainless steel was used as cathode, no spacers were included in theseparator envelope and the anode was mesh platinum screen measuring 2(5.1 cm) X 4 inches (10.2 cm).

The solution treated was a sample of an actual industrial effluent Ithad previously been determined that the composition of this waste streamfluctuated over a wide range, depending upon the particular plantoperations in progress at any given time. However, the sample testedhere had an initial soluble copper content of 15.5 ppm, a pH 2.8 and achloride content of approximately 900 ppm. There were traces of otherheavy metals, e.g., Fe, Ni and Cr, and a heterogeneity of dissolvedorganics, each in low concentrations. Sulfate was the principal anionpresent.

After filtering to remove suspended solids the solution was fed at 500cc/min (1.67 cm/sec superficial velocity) in repetitive passes to thecell supplied with applied current as listed:

TABLE 5 Treatment of Industrial Effluent A" Pass Current Copper Conc'n.

No. (ma) (ppm) EXAMPLE VI The cell of Example V was operated in the samemanner as described for Example V but used to treat a sample of adifferent actual industrial plant effluent B. This aqueous effluent wasvariable and heterogeneous in composition, but, as received for test,contained 48.5 ppm of copper, an approximately equal content of iron andhad a pH 1.5 due largely to the presence of sulfuric acid.

After a period of aeration to remove dissolved sulfur dioxide, thesolution was pumped to the cell in repetitive passes at a rate of 500cc/min (i.e., 1.67 cm/sec superficial velocity). The current fed duringeach pass, the effluent copper concentrations and the attendantcoulombic efficiencies are tabulated as follows:

Table 6 Treatment of Industrial Effluent 8" Pass Current Voltage CuConcn. Efficiency No. (ma) (volts) (ppm) In order to determine whatdisposition was made of 30 the iron, Example VII was conducted.

EXAMPLE VII The cell of Examples V and VI was used to treat industrialeffluent B in the manner of mode 1 (FIG. 6A). Here the object was toremove the toxic copper ions from the solution selectively, leaving theiron in solution for either subsequent treatment or discharge. Theresults of two tests, each consisting of one pass through the extendedsurface cathode cell, are as fol- This Example shows that the extendedsurface cathode was effective over a current range of at least 2:1 asregards selective removal of copper without accompanying iron removal.No significance is attached to the fact that effluent iron content isslightly higher than influent iron content, as this could be due toanalytical difficulties.

Hydrogen gassing at the cathode is, however, a natural consequence ofthe relatively current supply rate of Test 8-1, and it is possible thatthe generation of hydrogen bubbles screened the cathode, making itsomewhat less effective than in Test B-2 as regards copper removal. Inany case, considering both Tests together, it is clear that there existsan optimal current for a particular solution processed in a particularcell. In this Example, it is believed that this optimal current liesbelow the Test B-2 level rather than above it, because the coulombicvalue for the copper content of the asreceived effluent B is 660 ma.

EXAMPLE VIII The cell of Example 111 was operated in mode 2 (FIG. 68),that is, the solution treated was circulated from solution accumulationvessel 60' through cell 47", and thence back to vessel 60', which latterwas well-stirred during the test. Samples of solution were withdrawn viastopcock 63' for analysis at preselected time intervals denoted.

The solution employed here was 2400 cc of a synthetic lead-containingwaste prepared by dissolving sufficient Pb(NO in water to bring the leadconcentration to 23 ppm. The pH of the solution was adjusted to 1.33with nitric acid. The solution flow rate through the cell during thetest was maintained at 300 cc/min, while the applied current wasmaintained at 500 ma. The progressive lowering of lead content is shownby the following.

Table 8 Electrolysis of Synthetic Lead Wastes Elapsed Time LeadConcentration (min) (ppm) EXAMPLE IX Table 9 Electrolysis of SyntheticMercury Waste Elapsed Time Mercury Concentration min) (pp EXAMPLE X Thecell of Example 111 was operated as in Example VIII. The initial aqueousfeed solution consisted of 2,400 cc of acidic silver nitrate solutioncontaining 20 ppm of silver at a pH of 1.35 as adjusted with sulfuricacid. The solution was circulated at a rate of 300 cc/min and theapplied current was 500 ma. The reduction of silver content achieved wasas follows:

Table 10 Electrolysis of Synthetic Silver Waste Table II Electrolysis ofFilm Wash Water Elapsed Time Silver Concentration (min) (ppm) EXAMPLEXII The cell of Example 111 was operated as in Example V111. The initialfeed solution was a sample of an actual aqueous industrial effluent C.Experience showed that the effluent composition could be expected tofluctuate, as do most industrial wastes, but this sample contained 425PPM of copper at a pH of 0.3. There were negligible suspended solidspresent, while the dissolved solids were in the range of 10 to 15percent. The low pH resulted from a mixture of hydrochloric and sulfuricacids. This solution was circulated through the cell at 300 cc/min withan applied current of 1,000 ma.

The reduction in copper content is shown in the plot of P16. 8.

EXAMPLE XIII A cell was constructed without an extended surface cathodeto demonstrate the comparative advantages of such electrodes.

Thus, two platinum foil electrodes, each 5 mils (127 microns) thick withdimensions of 2 cm X 5 cm, were positioned longitudinally 1.2 cm apartand facing each other in a 1 inch dia. glass tube. The liquid to betreated was circulated through the tube and past both electrodes.

The cell was operated in the manner of Example Vlll (FIG. 6, mode 2)with a recirculation rate of 300 cc/min and an applied current of 200ma. The aqueous solution under test was 200 cc of pH 1.7 containinginitially 105 ppm of copper from copper sulfate and 10 cc/liter ofphosphoric acid. Copper was removed from solution, but at lowefficiency, as shown in the following Table:

Table 12 Electrolysis with Foil Electrodes Elapsed Time CopperConcentration Current Efficiency (min) (pp EXAMPLE XIV To demonstrate aprocess application of extended surface electrodes, the system shown inFIG. 2 was assembled.

Five cells constructed as described in Example 1 were assembled invertical stack or column formation, one on top of the other, to make upthe ESE unit. The liquid in treatment was pumped into the bottom of thecolumn and exited out the top. A line from the top of the ESE unitcarried the effluent back to the feed (hold up) tank 45 where the copperconcentration was adjusted to maintain a constant feed composition inthe cell stack. Each of the five cells was powered by a separate D-Cpower supply, and provision was made for sampling the liquid streambetween adjacent cells in the stack.

The aqueous solution treated was an acidified dilute solution of coppersulfate containing 20 ppm of copper at a pH 2.5. Sulfuric acid was usedfor pH adjustment. The solution resistivity was adjusted to 40ohmcentimeters by the addition of sodium sulfate. This solution waspumped through the ESE unit at a rate of 3 gpm (1 1.7 liters/min),giving a superficial fluid velocity of 9.7 cm/sec. Thus, for the fivecells in series, each cell having a working length in the flow directionof about 7 cm, the contact time for the solution treated wasapproximately 3.6 seconds.

Each cell was operated at constant current by its individual powersupply. The applied voltages were low; for example, throughout the 14hours of the run, the voltage to the first cell never exceeded 5.25volts, even though the current 17 amps. Because lower currents wereapplied to the other cells, their voltages were correspondingly lower.

The effluent copper concentration was measured at intervals for both thefeed solution and for the solution exiting from each of the five cells.The results are set out in appended Table 13, along with the pressuredrop (AP) across the cells. Thus, the column headed Cell 1 reports thecopper concentration, in ppm, of the solution exiting the first cell,the column under 11 gives the same information for fluid exiting thesecond cell, etc. The pressure drop across the first cell is AP whereasthe total pressure drop across all five cells is Al, Of course, thepressure drops increase with passage of time due to the gradualaccumulation of copper in the cathode structures.

Table 1 3 M ULTl-CELL ELECTROLYSIS Flow Rate: 3.0 gpm Pressure Drop, psi

Feed Cell 1 Cell 11 Cell 111 Cell lV Cell V AP, AP,

Current Amp. l7 1 l 8 5 3.5

W Elapsed Time. hrs.

2 20.9ppm 15.5ppm l 1.5ppm 7.7ppm 5.9ppm 4.lppm 1.25 5.75 4 20.2 16.09.1 7.5 4.3 3.0 1.5 6.25 6 19.5 14.2 8.4 6.3 4.0 2.4 2.0 7.0 8 20.6 15.811.4 9.3 5.8 4.1 2.5 8.2 10 19.7 14.5 10.3 7.9 5.0 4.0 2.8 8.8 12 20.016.5 10.3 8.3 4.1 3.1 3.2 9.4 14 20.4 16.4 11.5 8.2 4.5 3.1 3.5 10.0

I Pressure drop across first cell. "Pressure drop across all five cells.

At this point in the operation of the system, the input of freshsolution ppm of Cu content) was halted and the power supplied switchedoff. The column of cells was drained, flushed with water for 30 seconds,drained again, then contacted with an acidic leaching solution to removethe accumulated copper from the stainless steel cathodes.

For the leach operation, a volume of 9.0 liters of approximately 20percent nitric acid was used. It had been used once previously for asimilar leaching operation, so that its copper content was 22,800 ppm(2.28 percent) as initially fed to the cell column. The leachant wascirculated through the cell group assembly at a rate of approximately 6liters/min for a period of 24 minutes. At that point, no further tracesof copper were seen to remain in the cell column and leaching wasdiscontinued, the column was drained, then flushed with water for 30seconds and drained again.

Flow of the synthetic aqueous waste solution was resumed through thecell column and electrical power again supplied to the individual cells.The copper content of the leachate was determined to be 40,700 ppm.Nitrate analysis of the solution showed the leaching operation to beessentially stoichiometric.

The cycle hereinabove described was thus 14 hours of extended surfaceelectrode operation; drain, leach, etc., approximately 30 minutes.

The leachate was then circulated at the relatively low flow rate ofabout 1 liter/min from leachate tank 51, FIG. 2, to recovery unit 54 andthence back to the leachate tank, which latter was a 5 gallon (19 liter)polyethylene jug.

Recovery unit 54 was'a conventional electrolytic copper recovery cellhoused ina methyl methacrylate polymer tank 15.2 cm wide X 15.2 cm deepX 36.8 cm' long. The leachate was pumped into the tank at one end nearthe bottom and flowed over a weir 12.7 cm above the tank bottom beforeexiting at the opposite end of the tank. A multiplicity of alternatedflat anode and cathode plates constituted the electrodes of the recoverycell, these being hung transverse the tank from polymeric holders, sothat the plates were at 90 to the general direction of solution flow.Two cathode plates separated one from another a distance of 1.9 cm andimmersed to a depth of 10.2 cm. The anodes were mesh (31.5 mesh/cm)platinum screens made up from filaments 107 um dia. The cathodes were 16gauge type 316 stainless steel plates.

The three anode screens were connected in parallel electrically, as werealso the two cathode plates.

Power was supplied to the electrode of recovery unit 54 from a separated-c supply at a rate of 22 amperes. This corresponds to a cathodiccurrent density of 50 ma/cm After 5 hours of operation, a total of 99gms of copper had been deposited on the two cathodes. The copperconcentration of the leachate had been reduced to 29,800 ppm. Overall,the electrical efficiency for recovery of copper from the leachate was76 percent.

In similar test conducted in the same general manner, but withmechanical agitation of the solution, electrical efficiencies as high as98 percent were obtained. Such solutions, after copper depletion, areready, after appropriate NHO make-up addition, for reuse in the nextleaching operation.

. Example XV Industrial effluent 6", an aqueous solution of a cationictype red dye, containing approximately 300 ppm of that dye in additionto 0.1 percent by weight of glycolic acid, 2 percent dimethyl formamideand ppm of chloride ion, was circulated through an energized extendedsurface area cell for the purpose of decolorizing the solution.

For this test an extended surface cathode cell was as sembled asfollows: the anode was 80 mesh (31.5 mesh/cm) woven platinum screen madeup from individual filaments 0.0042 inch (107 pm) dia. The anode was asingle sheet measuring 2.5 (6.4 cm) X 5.0 inch (12.7 cm). The cathodewas 15 layers of knitted 0.002 inch (50.8 um) 2-filament, type 316stainless steel mesh 2.5 (6.35 cm) X 5.0 inch (12.7 cm), weighing 14.3gms. A single piece of Vexar screen, of the type described for ExampleIII, was placed beside the described anode into a Tyvek envelope, of thetype described in Example 111, the whole subassembly being placed in a60 mm dia. whatman extraction thimble cut off at the closed end to forman open cylinder. The

Whatman extraction filter is marketed by W. & R. Balston, Ltd.,Maidstone, Kent, England, and has the shape of a hollow cylinder closedat one extremity by a hemispherical end. It is made from heavy paperpulp, having a thickness of about one mm, and is used in standardlaboratory extractions by the Soxhlet technique. The cell separatorlayer was cut to fit smoothly once around the cylindrical cathode withno overlap at the ends. The Whatman filter served as a peripheral sealagainst electrolyte bypassing around the electrolyzing apparatus. Theentire assembly was placed inside a 1.0 inch (2.54 cm) inside diameterglass tube so as to give the configuation shown in FIG. 3.

To start this run, 400 cc of effluent G solution was placed in areservoir vessel 60, FIG. 6B, and recirculated (mode 2) through the cellhereinabove described at a rate of 300 cc/min (correspoding to asuperficial linear velocity of 1.0 cm/sec). Current to the cell was thensupplied at 500 ma, which required approximately 10 volts across thecell. Every hour, 5-10 cc samples of the solution in the reservoir 60were taken for spectrophotometric analysis by measuring the absorptionpeak at 525 mu. During the course of this experiment, the destruction ofthe red-colored dye was seen to occur visually. The data taken were asfollows:

Table 14 Decolorization of Industrial Effluent G An independentexperiment confirmed that destruction of the dye probably occurred atthe cathode; however, because of lack of knowledge of the precisemechanism of decolorization current efficiencies were not calculated.

EXAMPLE XVI A synthetic aqueous cyanide waste was circulated through anenergized extended surface anode cell to demonstrate the oxidativedestruction of cyanide ion through the following probable mechanism:

2 CN+ 4 OH 2 CNO'+ 2 H O 4 e, which reaction has a potential of 30 0.970v. vs. the standard hydrogen electrode (refer Standard Aqueous ElectrodePotentials and Temperature Coefficients at 25C. by A. J. de Bethune andN.A.S. Loud).

The synthetic waste was made by dissolving sodium hydroxide to aconcentration of0.0l N NaOH solution (pl-l 12) hydroxide to aconcentration of 0.01 N NaOH solution (pH 12). To this solution wasadded NaCN to a concentration of 274 ppm, which corresponds to 145 ppmcyanide ion content.

The extended surface area was fabricated as follows: The extendedsurface area anode was 80 mesh (31.5 mesh/cm) woven platinum screenfabricated from filaments 0.0042 inch (107 um) dia. Five layers of thisscreen, each formed by folding a 12 inch long piece back on itself toform a 6 inch length, were used for the anode, forming an assembly 2.5(6.35 cm) X 6 inch (15.2 cm) and weighing 59.5 gms. The cathode was madefrom a single piece of the same platinum screen measuring 2.5 (6.35 cm)X 6 inch (15.2 cm). A single piece of Vexar screen of the type describedin Exampale III was placed, as a spacer, along with the describedcathode, into a Tyvek envelope separator of the type described inExample III. The whole subassembly was placed in a Whatman extractionthimble of the type described in Example XV and cut to the dimensions ofthe subassembly in the manner hereinbefore described in Example XV. Theentire assembly was placed inside a 1.0 inch (2.54 cm) inside diameterglass tube to give a configuration such as shown in FIG. 3. At the startof this experiment, 1 liter of solution was placed in feed vessel 60 andrecirculated through the cell 47" in mode 2 (FIG. 68) at a rate of 100ml/min. (corresponding to a linear velocity of 0.33 cm/sec.). Current tothe cell was then supplied at 500 ma. Samples of 10-15 ml were taken viastopcock 63' at specific intervals for cyanide ion concentrationanalysis by titration with AgNO solution in the presence ofparadimethylaminobenzalrhodamine as indicator.

Table 15 Destruction of Cyanide in Synthetic Cyanide Waste ExperimentNo. 1

CN Concentration of Solution in Feed Vessel 60' (ppm) Elapsed Time fromStart of Current App] n. mins.

0 145 10 125 20 I29 40 I17 50 I15 116.5 H5 I00 112 A second experimentwas made under the same conditions on a solution prepared exactly asdescribed for Experiment No. 1 supra, but containing only 103 ppm CNion.

The data obtained was as follows:

Table 16 Destruction of Cyanide in Synthetic Cyanide Waste ExperimentNo. 2

CNConcentration of Solution in Elapsed Time from Start of Current Appln.

EXAMPLE XVII The cell of Example III was operated as in mode 2, FIG. 6B.The solution treated was non-aqueous, composed of dimethylformamide inwhich 53.4 gms/liter of tetraethyl ammonium perchlorate were dissolved.The solution had a resistivity of 98 ohm-centimeters. Copper sulfate wasadded to produce a copper concentration of approximately 20 ppm. The pHof this solution was measured initially, and readings of 8.5 and 8.8obtained; however, it is doubtful that these values are true pH levels,for the reason that only a relatively small number of hydrogen ionsprobably existed in the solution.

A total of 300 cc of the solution was circulated through the cell at aflow rate of 300 cc/min (superficial velocity 1.0 cm/sec). The appliedcurrent was 300 ma. Copper concentrations as determined by atomicabsorption were as follows:

Table 17 Electrolysis of Non-Aqueous (Dimethylformamide) SolutionElapsed Time Copper Concentration mlns. PP

EXAMPLE XVIII I The cell of Example XVI, with the extraction thimblewrapping replaced by a single wrap of Teflon fluorocarbon sheetmeasuring 0.031 inch (795 pm) thick and 2.5 inch (6.4 cm) wide, was usedto reduce an aqueous quinone solution of approximately 100 ppmconcentration to hydroquinone by making the extended surface electrodethe cathode.

The solution was prepared by dissolving 400 mgs. of quinone (firstdissolved in methanol) in four liters of distilled water, so that thefinal methanol content was 0.30 percent and the pH 5.5.

At the start of the experiment, one liter of the solution hereinabovedescribed was placed in feed vessel 60' (FIG. 6B, mode 2) andrecirculated through ESE cell 47 at a rate of 100 ml/min (correspondingto a linear velocity of 0.33 cm/sec). Current was supplied to the cellas 125 ma. Samples of l015 ml volume were withdrawn via stopcock 63 atpreselected intervals for spectrophotometric analysis. The gradualformation of hydroquinone was traced by measuring its characteristicabsorption peak at 2930 A.

The results obtained were as follows:

Table 18 (Experiment No. l)

The overall coulombic efficiency was 36 percent.

Another experiment was carried out under the same conditions ashereinabove described, except that the current supplied to the cell was250 ma.

Table 19 (Experiment No. 2)

The overall efficiency was 26.5 percent.

EXAMPLE XIX This Example demonstrates the necessity for spacers 20, 20and 20", as shown in FIGS. 1 and 3, respectively.

Seven cells of the design of Example 1 (FIG. 3) were inserted invertical stack formation within a tubular glass conduit with electricalconnections made to each as detailed in Example XIV supra. Through thisextended surface electrolysis unit, the solution treated was pumped fromthe bottom inlet with exit out of the top at a rate of approximately 3gallons/min.

The aqueous solution treated was an actual industrial plant wastecontaining a varying amount of copper ions in the range of 1 to 10 ppm.The solution was filtered and adjusted to a pH of usually about 3 beforeit was passed to the cells. Each cell was operated at a current densityappropriate to the copper level in the solution at the point of cellentrance, and the current supply ranged from about 2 to about 8amperes/cell, giving current densities from about 10-40 ma/cm The cellstack reduced the copper concentration of the solution treated by atleast percent. The stack was operated continuously for 600 hours withoutloss of performance, except for one cell in which a Vexar spacerslipped, apparently during assembly of the cell, which permitted theTyvek separator 23 (FIG. 1) to contact the platinum screen anode. It wasfound that the Tyvek separator degraded seriously at those points whereit contacted the anode, allowing the cell to short out before the 600hours operation was achieved.

The cause of the degradation is not known for certain; however, it couldhave been oxidation occurring at the anode surface.

Another problem to be safeguarded against is that of electrical shortingdue to metallic dendrite growth occurring on the cathode and extendingtoward the anode. Dendrite formation is somewhat unpredictable,depending as it does on the metal ion concentration of the treatedsolution, the existence of localized high current density paths betweenthe electrodes and, probably, other variables.

ln any case, dendrite penetration through the separator elements 23,23', FIGS. 1 and 3 respectively, must be prevented, as it has been foundthat electrical shortout dendrites can form in as brief a period as 45minutes when electrolyzing copper solutions containing from 500 to 1,000ppmCu.

As hereinbefore described, Tyvek 1058 has worked well as a separatormaterial, this being a spunbonded 5 sheet formed by the randomdistribution of very fine translucent continuous fibers (about 0.0002inch dia.) which are self-bonded by heat and pressure. The compositeTyvek 1058 is about 0.006 inch thick and constitutes approximately 30fiber layers. Under visual examination, each of these layers presentsaverage triangular openings defined by any three randomly orientedfibers presenting maximum openings measuring 0.0008 to 0.0016 inch.However, since there is a multiplicity of superposed layers, theeffective openings for ionic transport are equivalent to devious poresof approximately 0.00016 inch size. Separator structures, as described,have the appearance of translucent sized book paper and are strongenough to prevent dendrite penetration therethrough while stillpermitting good electrode-to-electrode ionic transport.

in comparison, a Tyvek 1621 sheet, which has been treated to possessopenings of about 0.012 inch maximum dimension visible to the unaidedeye, i.e., about 100 times larger than the effective size of Tyvek 1058sheet, did permit dendrite penetration and consequent electrical cellshort out.

What is claimed is:

1. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing provided, at the lower end, with an inletport for electrolyte introduction, and at the upper end with an outletport for discharge of said electrolyte, at least one of said electrodesbeing of uniformly reticulated open construction so as to oppose lowresistance to electrolyte flow therethrough and having an extendedsurface area over which a substantially uniform reactionproducingelectrical potential is maintained with respect to surroundingelectrolyte throughout the portion of said area in confrontation withthe electrolyzing area of the remaining electrode, and said remainingelectrode being disposed within an electrically insulator separatorenvelope closed on all sides except at the edge adjacent to said outletport fabricated from an electrolyte-inert material which is relativelyliquid-tight in construction so as to bar the ready passage of saidelectrolyte therethrough but which is permeable enough to permit ionicpassage between said electrodes.

2. An electrolytic cell comprising at least two inter functioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing according to claim 1 wherein said separatorenvelope is fabricated from an electrically insulative web consisting ofone of the group made up of ion exchange membranes, textile fabrics,paper and spunbonded products having a pore size barring penetration ofdendrites built up by metal depositing on the cathode electrode of saidinterfunctioning electrodes.

3. An electrolytic cell according to claim 2 wherein the permeability tosaid ionic passage is equivalent to a pore size of about 0.2 mil.

t. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing according to claim 2 wherein said separatorenvelope is a spunbonded high-density polyethylene having a density ofsubstantially l.6 oz./yd. (corresponding to 0.0054 gm/cm and 6 mils(corresponding to 152p.m) thick, made up of individual fibers in therange of 0.2 mil (corresponding to 5pm) diameter.

5. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing according to claim 1 wherein said uniformlyreticulated electrode having an extended surface area over whichsubstantially uniform reaction-producing electrical potential ismaintained with respect to said surrounding electrolyte throughout theportion of said area in confrontation with the electrolyzing area ofsaid remaining electrode is a composite made of a plurality of doublelayers of electrically conductive metal having good corrosion resistanceto said electrolyte, each of said double layers being opposite sides ofa flattened tubular knit formation fabricated from wire measuring in therange of about 2-4.5 mils (corresponding to a range of about 0.510.l 15mm) diameter.

6. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing provided with a separator envelope accordingto claim 1 wherein said electrodes and said separator envelope aresufficiently flexible to be wound into a layered spiral compositefitting tightly within said leaktight housing and being closed offcentrally and peripherally to bar electrolyte bypass flow around saidelectrodes and said separator.

7. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing according to claim 1 wherein there isinterposed between said separator envelope and said remaining electrodea relatively thin spacer fabricated from material substantially inertwith respect to said electrolyte having a ribbed structure definingfull-length open vertical passages confronting said remaining electrodeand isolating said separator envelope from contact with said remainingelectrode.

8. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing according to claim 7 wherein said spacerconstitutes an extruded netting of high density polyethylene having athickness of between about l530 mils (0.76 mm) made up of about eightstrands/iunch with said strands intersecting each other in a diamondpattern.

9. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common trans verse level within a verticallyoriented leak-tight-housing according to claim ll wherein saidelectrodes are generally planar in form.

1. AN ELEECTROLYTIC CELL COMPRISING AT LEAST TWO INTERFUNCTIONINGELECTRODES DISPOSED AT A COMMON TRANSVERSE LEVEL WITHIN A VERTICALLYORIENTED LEAK-TIGHT HOUSING PROVIDED, AT THE LOWER END, WITH AN INLETPORT FOR ELECTROLYTE INTRODUCTION, AND AT THE UPPER END WITH AN OUTLETPORT FOR DISHARGE OF SAID ELECTROLYTE, AT LEAST ONE OF SAID ELECTRODESBEING OF UNIFORMLY RETICULATED OPEN CONSTRUCTION SO AS TO OPPOSE LOWRESISTANCE TO ELECTROLYTE FLOW THERETHROUGH AND HAVING AN EXTENDEDSURFACE AREA OVER WHICH A SUBSTANTIALLY UNIFORM REACTION-PRODUCINGELECTRICAL POTENTIAL IS MAINTAINED WITH RESPECT TO SURROUNDINGELECTROLYTE THROUGHOUT THE PORTION OF SAID AREA IN CONFRONTATION WITHTHE ELETROLYZING AREA OF THE REMAINING ELECTRODE, AND SAID REMAININGELECTRODE BEING DISPOSED WITHIN AN ELECTRICALLY INSULATOR SEPARATORENVELOPE CLOSED ON ALL SIDES EXCEPT AT THE EDGE ADJACENT TO SAID OUTLETPORT FABRICATED FROM AN ELECTROLYTEINERT MATERIAL WHICH IS RELATIVELYLIQUID-TIGHT IN CONSTRUCTION SO AS TO BAR THE READY PASSAGE OF SAIDELECTROLYTE THERETHROUGH
 2. An electrolytic cell comprising at least twointerfunctioning electrodes disposed at a common transverse level withina vertically oriented leak-tight housing according to claim 1 whereinsaid separator envelope is fabricated from an electrically insulativeweb consisting of one of the group made up of ion exchange membranes,textile fabrics, paper and spunbonded products having a pore sizebarring penetration of dendrites built up by metal depositing on thecathode electrode of said interfunctioning electrodes.
 3. Anelectrolytic cell according to claim 2 wherein the permeability to saidionic passage is equivalent to a pore size of about 0.2 mil.
 4. Anelectrolytic cell comprising at least two interfunctioning electrodesdisposed at a common transverse level within a vertically orientedleak-tight housing according to claim 2 wherein said separator envelopeis a spunbonded high-density polyethylene having a density ofsubstantiaLly 1.6 oz./yd.2 (corresponding to 0.0054 gm/cm2), and 6 mils(corresponding to 152 Mu m) thick, made up of individual fibers in therange of 0.2 mil (corresponding to 5 Mu m) diameter.
 5. An electrolyticcell comprising at least two interfunctioning electrodes disposed at acommon transverse level within a vertically oriented leak-tight housingaccording to claim 1 wherein said uniformly reticulated electrode havingan extended surface area over which substantially uniformreaction-producing electrical potential is maintained with respect tosaid surrounding electrolyte throughout the portion of said area inconfrontation with the electrolyzing area of said remaining electrode isa composite made of a plurality of double layers of electricallyconductive metal having good corrosion resistance to said electrolyte,each of said double layers being opposite sides of a flattened tubularknit formation fabricated from wire measuring in the range of about2-4.5 mils (corresponding to a range of about 0.51- 0.115 mm) diameter.6. An electrolytic cell comprising at least two interfunctioningelectrodes disposed at a common transverse level within a verticallyoriented leak-tight housing provided with a separator envelope accordingto claim 1 wherein said electrodes and said separator envelope aresufficiently flexible to be wound into a layered spiral compositefitting tightly within said leak-tight housing and being closed offcentrally and peripherally to bar electrolyte bypass flow around saidelectrodes and said separator.
 7. An electrolytic cell comprising atleast two interfunctioning electrodes disposed at a common transverselevel within a vertically oriented leak-tight housing according to claim1 wherein there is interposed between said separator envelope and saidremaining electrode a relatively thin spacer fabricated from materialsubstantially inert with respect to said electrolyte having a ribbedstructure defining full-length open vertical passages confronting saidremaining electrode and isolating said separator envelope from contactwith said remaining electrode.
 8. An electrolytic cell comprising atleast two interfunctioning electrodes disposed at a common transverselevel within a vertically oriented leak-tight housing according to claim7 wherein said spacer constitutes an extruded netting of high densitypolyethylene having a thickness of between about 15-30 mils (0.76 mm)made up of about eight strands/iunch with said strands intersecting eachother in a 90* diamond pattern.
 9. An electrolytic cell comprising atleast two interfunctioning electrodes disposed at a common transverselevel within a vertically oriented leak-tight housing according to claim1 wherein said electrodes are generally planar in form.